Flight management systems are known from the prior art that are designed for preparing a flight plan, then closed-loop controlling the aircraft over the latter.
During the preparation of the flight or during a re-routing, the crew inputs their flight plan into a dedicated computer: the FMS (Flight Management System).
FIG. 1 shows the various components of an FMS having the functions listed hereinbelow, described in the standard ARINC 702 (Advanced Flight Management Computer System, December 1996): they normally provide all or part of the functions for:                Navigation LOCNAV, 170, for performing the optimum localization of the aircraft depending on the geo-localization means (GPS, GALILEO, VHF radio beacons, inertial guidance systems);        Flight plan FPLN, 110, for inputting the geographical elements forming the skeleton of the route to be followed (departure and arrival procedures, waypoints, airways);        Navigation database NAVDB 130, for constructing geographical routes and procedures using data included in the databases (points, beacons, interception or altitude legs, etc.);        Performance database, PRF DB 150, containing the aerodynamic and engine parameters of the aircraft.        Lateral trajectory TRAJ, 120: for constructing a continuous trajectory based on the points of the flight plan, complying with the aircraft performance characteristics and the confinement constraints (RNP);        Predictions PRED, 140: for constructing a vertical profile optimized over the lateral trajectory;        Guidance, GUID 160, for guiding the aircraft in the lateral and vertical planes over its 3D trajectory, while at the same time optimizing the speed;        Digital data link DATALINK, 180 for communicating with the control centres and the other aircrafts.        
A flight plan is composed of a list of “legs” in the AEEC ARINC 424 format. A leg consists of a termination (which may be of navigation point or “waypoint”, a termination altitude, an interception with another leg, a beacon radial location, a distance with respect to a beacon) and of a path to be followed in order to arrive at the termination (orthodromic route, loxodromic route, imposed arrival heading for example). A flight plan is generated starting from procedures and from points, stored in the navigation database 130, structured according to the aforementioned ARINC 424 standard. The procedures are composed of a set of legs. These digital procedures are produced from data supplied by the states, corresponding to the procedures in force in the air space being traversed. For example, in order to construct a flight plan, the pilot chooses various procedures indexed by a name, and various points. The FMS then extracts these procedures and points from the database, then carries out an a concatenation of the successive procedures in order to generate the flight plan.
A flight plan may be used by the FMS in order to perform the calculation of the trajectories and of the predictions, and in order to closed-loop control the aircraft.
In a flight plan, the prior art mentions 2 particular points:                The next waypoint, called “TO waypoint”, corresponding to the first leg in front of the aircraft        The preceding waypoint, called “FROM waypoint”, corresponding to the last point which has been passed                    (thus, when the aircraft passes a “TO waypoint”, it becomes FROM and the next point the TO (often called “NEXT waypoint” becomes the new “TO waypoint”.                        
Thus, using the flight plan defined by the pilot, the lateral trajectory is calculated by the FMS. On this lateral trajectory, the FMS optimizes a vertical trajectory (in altitude and speed), taking into account any constraints on altitude, on speed, on time.
One example of vertical trajectory 20 is given in FIG. 2. A vertical trajectory is illustrated by a evolution of the altitude h as a function of a curvilinear abscissa x along the trajectory.
A vertical trajectory is a valuable prediction tool for the crew. The vertical predictions carried out by the FMS are made based on the initial data:                Aircraft current state (altitude, speed, position, on-board fuel and weight of the aircraft, etc.)        Flight plan present in front of the aircraft        Current Guidance mode (“managed” or “selected”—see below)        
The Predictions are calculated from one to the next along the pre-calculated lateral trajectory, and up to the end of the flight plan, for example by integration of the equations for the dynamic characteristics of the aircraft, with integration steps sizes designed to obtain the correct precision. These predictions typically calculate:                The predicted altitude of passage        The predicted speed of passage (from this, the predicted time is deduced)        The predicted wind at the waypoint        The predicted remaining fuel (and hence the weight of the aircraft).        
According to the prior art, the complete calculated vertical profile is broken down into three phases:                a climbing phase 10 from the departure airport to a first altitude flight level is calculated starting from the takeoff runway and up to the point denoted “Top of Climb” (T/C), which corresponds to reaching the start of the cruising flight level. This first part is determined by integrating the dynamic equations (reference needed) along the lateral trajectory and in the forward direction (predictions starting from the runway up to the T/C): these are then referred to as “Forward” predictions.        a cruising phase 11 consisting of a succession of altitude flight levels to be reached and associated altitude change points, denoted “steps”, generally localized in curvilinear abscissa x along the vertical profile, for example with respect to the remaining distance to be travelled in order to arrive at the destination (“distance to destination”). The cruising path is calculated from the T/C to the point of start of descent, denoted “Top of Descent” (T/D) which corresponds to the end of the cruising phase. This part is determined by integrating the dynamic equations along the lateral trajectory and in the forward direction (predictions starting from the T/C up to the T/D). The predictions are also denoted “Forward”.        a descent phase 12 from the last altitude flight level to the arrival airport calculated from the T/D to the final destination. It is calculated in 2 parts.        
A first part consists in calculating a descent profile Prof (altitude/speed) by starting from the destination and by integrating in a reverse direction up to the end of the cruise path, denoted “backward” predictions. This profile Prof is fixed in order to guarantee that the aircraft following it will indeed finish its flight on the runway (in altitude/speed). This calculation allows the time and fuel consumed to be determined “as a countdown”, together with the wind.
A second part is composed of “forward” predictions starting from the aircraft state up to the destination. They comprise altitude, speed, time, fuel and wind “forward” predictions by propagating the aircraft state.
As long as the aircraft is not in the descent phase (i.e. as long as it is on the climbing and cruising trajectory), the descent profile and the descent predictions are identical (i.e. the aircraft is predicted along the profile), as long as the profile is flyable.
With regard to the closed-loop control of the aircraft, there exist several flight plans managed by the FMS. The active flight plan is the flight plan over which the FMS is able to guide the aircraft when it is coupled to the automatic pilot. The effective closed-loop control of the aircraft over the active flight plan is obtained by coupling with the automatic pilot. The automatic guidance mode of the aircraft over the active flight plan is also known by the expression “managed guidance mode”.
Situations exist in which the aircraft is not closed-loop controlled over the active flight plan. For example, the air traffic controller situated on the ground may have cause to request the aircraft to leave its flight plan, for example in order to ensure the correct separation of the aircraft. In the example illustrated in FIG. 3, the aircraft 101 is in a descent phase or downstream of the T/D, and it has left its pre-calculated descent profile Prof for any given reason (ATC setpoint, effects of the wind, etc.).
When the aircraft leaves its flight plan, the pilot switches into a mode referred to as “selected guidance mode”, corresponding to a manual vertical piloting mode, again via the automatic pilot PA.
In this case, the FMS calculates predictions by considering that the aircraft always immediately rejoins the lateral and vertical trajectory, according to a predetermined hypothesis.
Typically, if the aircraft is not on its lateral trajectory (i.e. it is shifted with respect to this trajectory), a lateral trajectory for immediately rejoining it is calculated, according to pre-established hypotheses. For example, an FMS may take as hypothesis an orthodromic line between the aircraft and the “TO waypoint”, another may take as hypothesis a path for rejoining the leg formed by the “FROM—TO” with an interception at 45°, or at 90°, to this leg or the shortest distance that gets back to the TO, while at the same time incorporating the turn to be made, or else rejoining by the shortest route to the flight plan (rejoining the flight plan is not then necessarily carried out at the TO waypoint.
Also typically, if the aircraft is not on its vertical trajectory (i.e. it is above or below), an immediate rejoining path is calculated also according to pre-established hypotheses.
For example, for the descent phase, and for an aircraft under its vertical trajectory, an FMS may predict that the aircraft will remain at one flight level (i.e. at a constant altitude) until it intercepts the vertical trajectory in question. Another FMS may have a hypothesis for descending at a low angle or at a low vertical speed in order to also intercept the trajectory.
Again for the descent phase, and for an aircraft above its vertical trajectory, an FMS may predict that the aircraft will dive by adopting a predetermined altitude, greater than the current altitude. Another may predict rejoining with a given aerodynamic slope, steeper than the current slope, following this until it intercepts the vertical trajectory.
The drawback of this situation is that the pre-established hypotheses of the FMS for vertical rejoining only rarely correspond to the manual vertical guidance mode. The integration of the equations according to the pre-established mode therefore gives erroneous results with respect to the reality.
The manual vertical guidance mode is characterized by an altitude setpoint or target altitude, denoted “Altitude clearance”, chosen by the pilot, towards which the aircraft is directed. In order to reach it, several piloting modes exist:                constant vertical speed managed, denoted VS for “Vertical Speed”,        constant angle managed denoted FPA for “Flight Path Angle”,        constant altitude managed, denoted ALT for Altitude,        thrust managed, denoted “Thrust” or “OPEN”.        
In reality, the crew only fly for a certain time with the current manual vertical mode because, at a given moment, this current mode will be modified in order to follow the flight plan (and its altitude constraints). The return to a guidance mode closed-loop controlled to the flight plan is commonly called “de-selection”.
In the situations for which the pilot has come out of the closed-loop controlled (“managed”) guidance mode and the aircraft is in manual guidance (“selected”) mode and hence no longer follows its flight plan, this poses several problems:
First of all, the aircraft is still obliged to comply with a certain number of altitude constraints associated with its flight plan remaining to be flown (this is referred to as the flight plan “in front of” the aircraft).
These altitude constraints are each characterized by an altitude AO to be complied with associated with a time, expressed by a curvilinear abscissa x.
The constraint may be of the type:                “at”: the aircraft must reach the altitude AO for a given x0, symbolized by two top-to-tail triangles,        “at or above”: the aircraft must go above the altitude AO for a given x0, symbolized by a head-up triangle,        “at or below”: the aircraft must go below the altitude AO for a given x0, symbolized by a head-down triangle.        “Window”: the aircraft must go between 2 altitudes at the given point.This type of constraint corresponds to a “at or above” and a “at or below” at the same point        
The pilot flying in “selected” mode is in a state of uncertainty with respect to the compliance with these constraints. Indeed, the predictions performed by the FMS become erroneous, because they do not take into account the change of guidance mode and do not correspond to how the aircraft is effectively flying, as explained hereinabove.
One aim of the invention is to overcome the aforementioned drawbacks with the idea of determining a switch-over vertical point from which the pilot can go from manual mode to the mode guided by the FMS while complying with the constraints that are imposed on him/her.